1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with a system for simultaneously generating a plurality of tone channels each having an ensemble effect.
2. Description of the Prior Art
A realistic electronic simulation of an air driven pipe organ requires the generation of musical waveshapes for a number of families of tones each having the same approximate fundamental frequency for the pipes in each family corresponding to the same instrument keyboard switch. The fundamental frequency should be slightly different for each tone family so that the desired ensemble effect is achieved. A second requirement for the ensemble effect is that an exact tuning of the notes within a given tone family is not a desirable goal. Instead the tuning of the individual notes should vary from the true frequency by some random tuning error which is typical of the tuning errors found in an air driven pipe organ.
Digital tone generators have been disclosed which attempt to replicate either pipe organ tones as well as the tones produced by the conventional acoustic musical instruments. Digital tone generators are attractive systems because they can be implemented economically with relatively low cost microelectronic circuitry.
In the musical instrument described in U.S. Pat. No. 3,515,792, musical tones are produced by storing a digital representation of a waveshape characteristic, e.g. of an organ pipe tone, and repetitively reading out this stored waveshape at a selectable clock rate determining the fundamental frequency of the produced note. Stored in the waveshape memory are the actual amplitude values at a plurality of sample points. A frequency synthesizer produces a clock signal at a rate determined by the note selected on the instrument's keyboards. The stored amplitudes are read out of the memory repetitively at the selected clock rate to generate the selected musical tone.
In the musical instrument described in U.S. Pat. No. 3,809,786 musical tones are produced by computing, in real time, the amplitudes at successive points of a waveshape and converting these amplitude points to musical tones as the computations are performed. A conventional discrete Fourier transform is implemented to compute each amplitude point from a stored set of harmonic coefficients and a selected value of a frequency number which establishes the period of the generated musical waveshape. At each timing signal furnished by a logic clock, the frequency number is added to the contents of an accumulator. The number contained in this accumulator corresponds to the time variable in the output musical waveshape. In between the times that the frequency number is added to this accumulator, the accumulator content is successively added to a second adder-accumulator for a number of times which is equal to the maximum number of harmonic coefficients in a set stored in a harmonic coefficient memory. After each addition to the second accumulator, the accumulated value is used to address a trigonometric function value from a sinusoid table. Each read out trigonometric value read out from the table is multiplied by a harmonic coefficient whose order is associated with the number of times the accumulated frequency number has been added to the accumulator in the second adder-accumulator. The individual products are summed for the number of harmonic coefficients in the stored set to generate the desired waveshape amplitude point. The ADSR envelope amplitude modulation is obtained by scaling the set of harmonic coefficients as they are read out of memory and before they are multiplied by the corresponding read out trigonometric function values.
In the musical instrument described in U.S. Pat. No. 4,085,644 musical tones are produced by computing (not in real time) a master data set, transferring the master data set to buffer memories, and repetitively converting in real time the contents of the buffer memories to musical tones. The master data set is created repetitively and independently of the tone generation by computing a discrete Fourier transform algorithm using stored sets of harmonic coefficients. A time shared digital-to-analog converter transforms the data read out from the buffer memories to tone channels and provides ADSR amplitude modulation.
It is an object of the present invention to produce musical tones having an ensemble effect corresponding to that produced by several independent ranks of organ pipes. It is a further object of the present invention to produce musical tones of the celeste family of tones.